A DC shop on one of the busiest retail strips has installed a “Mosquito” device in order to drive away teenagers who often loiter and fight there. The unit emits an annoying beeep beeep beeep at 17.5 kilohertz, which is at the upper range of audibility for 13-25 year olds. The business owners like it, but one passerby remarked, “It’s classism and ageism. And it’s sad.”